This invention relates in general to AC (alternating current) Power Control; and more specifically to the simple control and distribution of AC power to heater loads typically found in X-ray film processing equipment, dual laminator, roller heaters, splicer heaters, dual fuser roller heaters, and other temperature controlled heating equipment. The technique of the invention reduces the AC line voltage fluctuations caused when high power is applied to the heaters and provides power management options and temperature control.
New European EMC regulations (FY2001), require that all electrical equipment meet the requirements for EN-61000-3-3, xe2x80x9cLimitation of voltage fluctuations and flicker in low-voltage systems for equipment with rated current equal to or less than 16 amperesxe2x80x9d. To comply with these new regulations, equipment that requires continuous switching of AC power for its normal operation may require significant modifications in the manner in which electrical power is distributed to the system over a period of time. The proportional control heating systems found in various types of photographic film processing equipment are continuously switching AC power on and off to precisely maintain the temperatures of the processor chemistry baths and film drying system. The typical control systems utilized in this equipment will switch the AC power frequently over a period of time, to maintain the required temperature to the loads for each heater sub-system in the film processor. To maintain a close temperature control in a heater sub-system within a film processor, it is not uncommon for the power to the heating elements to be switching over 100 times per minute. Limiting the voltage fluctuations and flicker in this type of equipment will require an AC power control system that can minimize the number and the magnitude of each switched AC power demand over the measurement time period as specified in EN-61000-3-3. A typical power distribution system of this type can be considerably more complex and may increase the amount of heat dissipated in the film processing equipment. A simple methodology is needed that provides a controlled amount of power to the heating elements as needed to reach and maintain the required temperature of the equipment heater sub-systems and to minimize the amount of switched AC power transitions.
Electrical equipment, that minimizes multiple surges of power from the AC main power, has been designed to convert the alternating current to direct current and store this energy in large banks of capacitors or batteries for distribution, as needed, by the intended load. This type of system generally provides an even flow of power to the equipment and greatly minimizes the effects of the demand on the AC main power for continuous surges of power. These power distribution systems are generally much more complex, with lower reliability, in their operation and require a much larger physical space than is required by a conventional power supply and heater control system. They will increase the amount of heat that is dissipated in the system because the efficiency of the power conversion from AC to DC power will always be less than 100%. There is a need for a solution to these problems.
According to the present invention, there is provided a solution to the problems discussed above.
According to a feature of the present invention, there is provided a design consisting of two AC power control devices, both of which are connected in series between the AC main power and a single heater element load. One device will allow only the positive portion of the AC power to pass through it to the heater load. The second device will only allow the negative portion of the AC power to pass through to the load. By properly controlling both these devices the system can provide 100%, 50%, or 0% of the AC power to be applied to the heater load. To minimize the number of times that the AC power is switched, as required to meet the requirements of EN 61000-3-3 (Flicker), the heating system duty cycle and period may also be adjusted as required by hardware logic or via a software algorithm in the controller unit of the AC power heating control system.
According to another feature of the present invention, there is provided an electrical heater control system with reduced flicker comprising: an electrical heating system; an AC (alternating current) power source for supplying AC current to said electrical heating system; an AC power control device connected between said electrical heating system and said AC power source for controlling the supply of AC current to said electrical heating system; and a controller unit coupled to said AC power control device for activating said device at zero crossings of said AC power to minimize flicker in AC current supplied to said electrical heating system from said AC power source.
The invention has the following advantages.
1. Reduces voltage fluctuations on the AC main power line.
2. Reduces electromagnetic interference caused from high switching currents.
3. Provides half power or fall power to the load with minimal power dissipation from the device.
4. Provides the ability to increment the AC sine wave power to a load from 0, xc2xd, and full power steps, or multiple power steps when used with multiple series or parallel heater loads.
5. Utilizes a device that switches on and off at the zero crossing of the AC sine wave, resulting in reduced EMI.
6. Requires a minimum of space to locate the half wave control devices.
7. Minimum cost of components to implement.
8. Increases reliability.
9. Reduces/minimizes temperature swings, providing increased temperature control.